package com.springmvc.controller;
...
import javax.validation.Valid;
import org.springframework.validation.BindingResult;

@Controller
@RequestMapping("/books")
public class BookController {
    ...

    @PostMapping("/add")
    public String submitAddNewBook(@Valid @ModelAttribute("NewBook") Book book, BindingResult result) { ➊

➋

        if (result.hasErrors())
            return "addBook";
        }

        MultipartFile bookImage = book.getBookImage();
        String saveName = bookImage.getOriginalFilename();
        File saveFile = new File("C:\\upload", saveName);
    ...
}